Crude oil pumped out from wells is normally directed to tanks for temporary storage before it is transported to refinery or manufacturing purposes. Regular examination has to be performed onto the crude oil storage tank to avoid accidental spill caused by the tank leakage. However, quiescent state of the crude oil in the storage tank prompts settlement of heavier fraction at the bottom of the tank forming a sludge layer thereof. The sludge mainly presented in a form of stable emulsion consists of long chain paraffin, asphaltenes, inorganics and water. Sludge layer must be removed from the tank before proper inspection can be performed. Apart from that, the formed sludge layer also reduces storage capacity of the tank and interfere connection in between the tank to the refinery. It has been reported as well sludge accumulation interrupts operation of floating roof of the storage tank especially when the crude oil level is low in the tank. Considering problems can be initiated by the presence of sludge layer in the tank, effort has been put up to devise better ways to remove the sludge layer. Still, the process is very costly and time-consuming even after years of research.
It is important to be noted that sludge removal process generally includes as well treatments such as separation and oil recovery from the sludge prior to disposal of the sludge. Discharge of improperly treated sludge can inflict great damage to the environment. Since sludge is adherent in a solid or semi-solid form, it is impossible to be channeled out from the tank via pressure pump. Physically removing the sludge using machines or human force is the most common applied approach that, sometimes, it may be assisted by melting the sludge using heated medium like steam. For example, Japanese patent application no. 8310589 and International patent application no. 03002275 disclose specially designed system for removing sludge physically from the tank. Such approach may be, able to clean the tank, while the process is time-consuming and labor-intensive. Even cleaning a single tank, it takes days to complete. Further, post treatment after removing the sludge physically, like centrifugation or filtration, always fails to clean all the oil residues off solid particles in the sludge and it is unlikely to dispose the sludge without polluting the surrounding environment.
Beside physical removal, re-suspending the sludge into an oil fraction is another option to reduce depth of the sludge inside the tank. Preferably, the oil fraction is introduced on top of the sludge under high pressure, while preheating or vortex may be applied to accelerate dissolution of the sludge back into the crude oil. For example, U.S. published patent 2008/0047871 and International Patent Application 2010019548 respectively describe sludge removal using high solvent power (HSP) crude oil and atmospheric residue fraction of high solvency dispersive power crude oil. Other U.S. Pat. Nos. 4,945,933, 5,019,016 and 5,460,331 disclose different apparatus equipped with a circulator coupled to rotatable nozzles to create jet flow for dispersing sludge in crude oil tank. Though re-suspension or re-dispersion work well in discharging the sludge out from the tank, the discharged sludge of these patents may eventually deposit somewhere in the subsequent downstream processes and further treatment is needed to extract the oil before dumping the sludge. In view of this, United States published patent 2009/0173363 proposes deployment of filtration means to remove the solid particles along pumping out the crude oil suspension from the tank to avoid re-deposition of the sludge. At least two filtering means are disposed in the disclosed system to facilitate frequent filter exchange. Still, it is impossible to achieve near complete extraction of oil residue from the sludge through centrifuge or cyclone. While decomposing the sludge using biological agent is described in U.S. Pat. No. 6,069,002, the process requires a prolonged period in order to fully break down the sludge.
Some of the above mentioned approaches may have achieved significant results in sludge removal. Yet these approaches are far from cost and energy efficient. Ideally, the process or the composition used in sludge removal shall be able to drive recovery of the entrapped oil residues from the solid particle in a spontaneous mechanism without much machinery or energy intervention.